NOTES
4014 findings force us to take into consideration the possibility of the contribution of OH radicals to dechlorination. If the reactions 6, 7, and 8 are assumed, simple competition of these reactions leads to eq 9.
of dechlorination caused by OH radicals is expressed as ‘/@OH.
( 5 ) The solvated electron does not react with CHaOH in the presence of high NzO concentration, because of hap+ N%O>> keas- + C H ~ O H .
(6) T. Balkas, F. S. Dainton, J. K. Dishman, and D. Smithies, Trans. Faraday SOC.,62,81 (1966). 3.2 3.0 2.8
3d Transition Metal Complexes in Molten 1.2
3
Potassium Thiocyanate Solution
1.0
> Q
0.8
by H. C. Egghart
0.6 0.4
Energy Conversion Research Division, U.S . Army MobWu Equipmen Research and Development Center,Fort Belvoir, Virginia 86060 (Received November 18,196’88)
0.2 10-4
10-2
10-~
[CH,OH].
Figure 2. Effect of additive CHaOH on G(Nz), G(Ha), and G(Cl-) in t h e system of 1 X M ClCH&OO-2.3 X M NSO. 0, G(Cl-); A, G(Hz); 0, G(N2).
I
Figure 3.
OH OH
0.5
1
[CH,OHJ
/[CiCH,COO-].
1.5
2.0
A plot of l/G(Cl-) against [CHaOH]/[ClCHaCOO-].
+ ClCHzC00- +H2O + ClCHCOO-
+ ClCHzCOO+C1OH
+R
(R: radical) (7)
+ CHBOH+H2O + ’CH20H
1 G(C1-) G(0H)
(’ -I-
k13
-I-
(6)
(8)
k7[C1CHzC00-]
Figure 3 shows a plot of l/G(Cl-) against [CH30H]/ [C1CH2C00-]. A straight line supports the validity of the simple competition mentioned above. Since 0produced via reaction 2 is considered to show the same behavior as OH radicals, G(0H) of eq 9 should be taken to be 5.0 (GeaQ-= 2.8, GOH = 2.29. Then, Figure 3 leads to k6lk.l N 4.0 and lc8/k7 N 12. I n other words, it may be concluded that the degree of the contribution The Journal
of
Physical Chemdstry
Since 1956 adsorption spectroscopy has been used as a technique to study transition metal complexes in molten salts such as molten alkali halides and to a lesser extent low-melting alkali nitrate Only one investigation on transition metal species in molten potassium thiocyanate solution was carried out.6 I n that study results of preliminary character on Cr(III), Co(II), and Ni(I1) were obtained. Very low molar absorbances were reported which do not agree well in every case with the tentatively proposed structures of the complexes. The low molar absorbance ( E 15) reported for Cr(III), its spectrum indicating octahedral coordination, was not too surprising since d-d transitions in a system having a center of symmetry are Laporte-forbidden and gain some intensity only by distortion from a regular octahedral symmetry or by a vibronic mechanism. A tetrahedral field has no center of symmetry and d, p, and ligand orbitals can become mixed together to some extent. I n this way forbidden transitions become allowed to a certain degree.’ Therefore, tetrahedral complexes show much higher molar absorbances than octahedral complexes. I n view of this, particularly the molar absorbance of 86 reported for the maximum of the spectrum of Co(I1) solutions in molten potassium thiocyanate seemed too small for a tetrahedral species. The present investigation was carried out (i) to resolve these questions concerning the molar absorbances, (1) D. M. Gruen, Nature, 178,1181 (1956). (2) D. M. Gruen, J . Inorg. Nucl. Chem., 4,74 (1957). (3) D.M. Gruen, ”Fused Salts,” B. R. Sundheim, Ed., McGraw-Hill Book Co., Inc., New York, N. Y., 1964,Chapter 5,p 301. (4) G. P.Smith, ‘%MoltenSalt Chemistry,” M. Blander, Ed., Interscience Publishers, New York, N. Y., 1964,p 427. (5) D. M. Gruen, Quart. Rev., 19,349 (1965). (6) G. Harrington and B. R. Sundheim, Ann. N . Y . Acad. Sci., 79, 950 (1960). (7) C. J. Ballhausen and A. D. Liehr, J . Mol. Spectroac., 2, 342 (1958).
4015
NOTES (ii) to measure the absorptions also in the near-infrared region which should permit the interpretation of the Ni(I1) spectrum, and (iii) to study the spectra of other first row transition metal ions in molten potassium thiocyanate solution. This melt itself aroused interest because it has a considerably higher ligand field than the more widely studied alkali halide and nitrate melts. Experimental Section Certified reagent grade potassium thiocyanate was dried at 110" overnight and subsequently subjected to a thermal shock drying technique, described in detail in ref 8. The dried potassium thiocyanate samples were weighed and then melted under vacuum, and known quantities of water-free transition metal thiocyanates, chlorides, or sulfates were added. Part of these solutions were then transferred into quartz cells which were held a t temperatures above the melting point of potassium thiocyanate, usually 185", in a temperature-controlled sample cell holder. The absorptions of the solutions were compared with the absorptions of the pure solvent melt, using a Cary 14 recording spectrophotometer equipped with a water-cooled sample compartment. For the calculation of the molar absorbance it was necessary to know the volumes of the melts. These were determined from the weight of the melt using a value of 1.60 for the density of the potassium thiocyanate melt at 185". This value for the density was measured with a pycnometer. Results and Discussion Molten potassium thiocyanate is a useful medium for studies in the near-infrared, visible, and near-ultraviolet region of the spectrum. Its range of transparency in the near-infrared is far greater than that of water. Only very weak absorptions occur a t wavelengths shorter than 2300 mp. Work a t wavelengths below 320 mp is not possible because of strong absorption by the melt which is due to an internal transition in the thiocyanate ion.9 The intense absorptions beginning with Ni(I1) and V02+(lV)near 500 mp and with Co(I1) and Cr(II1) near 400 mp, shown on Figure 1, are chargetransfer bands. The same results were obtained when transition metal ions were added as thiocyanates, chlorides, or sulfates. This is understandable in view of the relatively high coordinating power of the thiocyanate group and the large excess in which this group was present. The results of the spectrophotometric work can be seen in Figure 1 and Table I. The absorption in the near-infrared region expected for octahedral Ni(I1) complexes was found and had a maximum a t 1170 mp (8547 cm-l) having a molar absorbance e of 13. Taking the ligand field strength parameter Dq as 854.7, the second band can be predicted to be a t the wavelength where it actually occurs. The first band of
1
WAVILtllQlH (np)
Figure 1. Absorption spectra of Ni(II), Co(II), Cr(lII), and VOZ+(IV) in molten potassium thiocyanate solution.
Cr(II1) was found a t 597.4 mp (16.739 cm-I) (e 191). With Dq = 1673.9 the second band can be predicted to be where the slight shoulder in the blue part of the spectrum can be seen. The good agreement between the observed spectra and the predictions of theorylOJ1 as well as their similarity with the spectra of Ni(I1) and Cr(II1) substituted in octahedral sites of host 1 a t t i ~ e s ' ~ J ~ leaves no doubt that these ions are octahedrally coordinated in molten potassium thiocyanate solution. Although nTi(I1) was found to be tetrahedrally coordinated in alkali halide melts,14 this would not be likely in potassium thiocyanate melts because the thiocyanate group exerts a considerably stronger ligand field than the chloride group and ligand field stabilizations in octahedral fields are particularly large for d a and d8 systems. The spectra found when Cr(II1) and Cr(I1) salts were dissolved in molten potassium thiocyanate were identical. Since the spectrum of Cr(II), a d4 system, should be entirely different, one must conclude the Cr(I1) was oxidized to Cr(II1). This conclusion gains support from the observation that Cr(V1) is reduced to Cr(II1) in molten potassium thi~cyanate.'~ Solutions of Co(I1) in molten potassium thiocyanate showed an intense band with a maximum a t 625 mp ( E 510), a shoulder around 585 mp and an absorption in the (8) E. Rhodes and A. R. Ubbelohde, Proc. Roy. Soc., A251, 156 (1959). (9) C. K. Jbrgensen, "Absorption Spectra and Chemical Bonding in Complexes," Pergamon Press, Oxford, 1962, p 196. (10) A. D. Liehr and C. J. Ballhausen, Ann. Phys. (N. Y.), 6 , 134 (1959). (11) L. E. Orgel, J. Chem. Phys., 23, 1004 (1955). (12) W. Low, Phys. Rev., 109,247 (1958); 105,801 (1957). (13) .More references can be found in C. J. Ballhausen, "Introduction to Ligand Field Theory," McGraw-Hill Book Co., Inc., New York, N. Y., 1962,p 238. (14) D. M. Gruen and R. L. McBeth, J . Phys. Chem., 63,393 (1959). (15) D. H. Kerridge and M. Mosley, Chem. Commun., 505 (1965).
Volume 73, Number 11 November 1069
NOTES
4016
Table I: Observed Absorption Maxima of Ni(II), Cr(III), Co(II), and V02* (TV) in Molten Potassium Thiocyanate Solution, Their Molar Absorbance E, and Some Assignments” Ni(I1)
1170 mp (8547 cm-l), E 13, *Azg ‘T*g(F)
710 mp (14,084 om-’), e 17, *Azg + *Tlg(F)
Cr(II1)
5974 mp (16,739 cm-I), 6 191, 4AAzs -c 4Tzs(F)
shoulder ~ 4 3 mp 0 (23,255 om-’), “2, 4Tig(F)
-,
+
Co(I1)
625 m p (16,000 cm-I), e 510 13,000 mp (7692 cm-l), E 64
shoulder -585 mp (17,094 cm-l)
VOZ+(IV)
800 mp (12,500 om-’),
613 mp (16,313 om-’), e 43
e 69 a
See ref 10, 11, 13, and 29,
near-infrared region. This spectrum was observed also with a number of Co(I1) solutions in nonaqueous solvents containing an excess of thiocyanate ions. This invariance of the spectrum in different solvents a8 well as other data indicated the formation of a tetrahedral Co(I1) thiocyanate species in organic solvents. A very similar spectrum was also found in investigations of solid KZ[Co(NCS)4] and [(CH3)4N]2[Co(NCS)4] and the acetone solution of the latter.18 X-Ray structure determinations showed that the lattice of Kz[Co(NCS)4] consists of [Co(NCS)#- tetrahedrons in which the thiocyanate groups are coordinated to cobalt ijia nitrogen.lg The great similarity of all these spectra with the spectrum of Co(I1) in molten potassium thiocyanate as well as the high molar absorbance indicates that in potassium thiocyanate melt solution Co(I1) is also tetrahedrally coordinated by four N-coordinated thiocyanate groups. I n addition to Ni(II), Co(II), and Cr(II1) also VOz+(IV) salts dissolved well in molten potassium thiocyanate and gave stable solutions. The first absorption band was found a t 800 mp ( E 69) and the second a t 613 mp ( E 43). Similar spectra were observed with many other vanadyl complexes in solution and in the solid ~ t a t e . ~ O -I ~ n ~their scheme for [VO(HZO)~]~+ and related complexes, Ballhausen and Gray assigned the first band to the transition 2Bz -t zE(I) and the second -3- Z B I . ~ The ~ energy of the to the transition z B ~ second band is 1ODq. This scheme is no more generally accepted since it was found that a t low temperatures the first band splits into four narrow band^.^^-^^ However, the second band was never found to split and using the energy of this band ligands could be properly ranked according to their ligand field strengths.28 The N-coordinated thiocyanate group generally causes a greater ligand field splitting than water. Consequently the -NCS group is ranked above the aquo group in the spectrochemical series.z9 Therefore it l 6 t n
The Journal of Physical Chemistry
was interesting to note that in molten potassium thiocyanate the ligand field strength of the -NCS group is about the same [observed with Ni(I1) and V02+(IV)] or slightly less [observed with Cr(III)] than the ligand field strength of H20 in aqueous solution. This observation led to a comparison of thiocyanate complexcs in different media. I n several investigations on solution and solid state spectra of [V0(NCS),la- complexes having organic cations the maximum of the second vanadyl band was found around 580 mp (17,241 cm-1).20-zz~z8 I n molten potassium thiocyanate solution it is at 613 mp (16,313 cm-l). Cr(II1) is known to be octahedrally coordinated a t all temperatures, and the positions of the absorption peaks in, for instance, molten alkali halide solution change very little when the temperature is raised from 400 t o 800°.30 However, between the 4Az, + 4Tz,(F)transition ( = lODq) of [Cr(NCS)6]3- in ordinary solutionsa1 and molten potassium thiocyanate solution there is a difference of about 1000 wave numbers as was found for [VO(NCS),I3-. I n studies on solids it was shown that the ligand field strengths in octahedral sites is influenced by cations in the second sphere e n v i r ~ n m e n t . ~I n~ the potassium thiocyanate melt potassium ions form the second sphere environment and their interaction with the thiocyanate group may cause the smaller ligand field strength of this group in the melt.
Acknowledgment. The author wishes to thank Dr. J. R. Huff, Chief of the Energy Conversion Research Division, for his interest in this work. (16) V. Gutmann and 0. Bohunovsky, Monatsh. Chem., 99, 751 (1968). (17) R. Lundquist, G. D. Markle, and D. F. Boltz, Anal. Chem., 27, 1731 (1955). (18) F. A. Cotton, D. M. L. Goodgame, M. Goodgame, and A. Sacco, J . Amer. Chem. SOC.,83,4157 (1961). (19) G. S. Zhdanov and A. V. Zvonkova, Zh. Fiz. Khim., 24, 1339 (1950). (20) J. Selbin, Chem. Rev., 65,153 (1965). (21) V. Gutmann and H. Laussegger, Monatsh. Chem., 99, 947 (1968). (22) V. Gutmann and H. Laussegger, ibid., 99,963 (1968). (23) V. Gutmann and H. Laussegger, ibid., 98,439 (1967). (24) C. J. Ballhausen and H. B. Gray, Inorg. Chem., 1, 111 (1962). (25) J. Selbin, T. R. Ortolano, and F. J. Smith, ibid., 2, 1315 (1963)’ (26) T. R. Ortolano, J. Selbin, and S. P. McGlynn, J . Chem. Phys., 41, 262 (1964). (27) J. Selbin, Coord. Chem. Rev., 1, 293 (1966). (28) 0. Piovesana and J. Selbin, J . Inorg. Nucl. Chem., 31, 433 (1969). (29) See, for example, T. M. Dunn, “Modern Coordination Chemistry,” J. Lewis and R. G. Wilkins, Ed., Interscience Publishers Inc., New York, N. Y., 1964, p 266. (30) D. M. Gruen and R. L. McBeth, Pure Appl. Chem., 6, 23 (1963). (31) C. I
